1. Field of the Invention
The present invention relates to a semiconductor substrate having a porous layer, a thin-film semiconductive member formed using the same, and a method for making them.
2. Description of the Related Art
The following studies have been performed in technical fields such as solar batteries. A silicon thin-film is formed on a porous layer provided on a silicon substrate which is made by cutting a single-crystal ingot, and then the thin film is separated from the substrate by cleavage of the porous layer.
It is preferable that the porous layer include at least two sublayers having different porosities in order to facilitate separation of the thin film from the substrate. Such a porous layer is formed by, for example, anodic oxidation of the surface layer of the substrate while varying a current density on the surface layer. The anodic oxidation means electrolysis of the substrate as an anode in an electrolytic solution containing hydrogen fluoride (HF). For example, anodic oxidation is performed by conducting a current flow with a current density of 1 mAcmxe2x88x922 for 8 minutes, a current density of 7 mAcmxe2x88x922 for 8 minutes, and then a current density of 200 mAcmxe2x88x922 for 4 seconds in an electrolytic solution of a 1:1 mixture of 50% hydrogen fluoride and ethyl alcohol (C2H5OH). The formed porous layer has a high-porosity sublayer interposed between two low-porosity sublayers.
The formation of the high-porosity sublayer, however, requires a large current density. As a result, distortion will occur in the porous layer, and hinders an improvement in crystallinity of the thin film formed thereon. A large current density also causes a nonuniform distribution of current density in the surface layer of the substrate. Thus, a high-porosity sublayer is unevenly formed in the porous layer. As a result, the thin film cannot be easily separated at some portions. Furthermore, such a current flow of a large current density requires a large electric power source.
It is an object of the present invention to provide a semiconductor substrate having a porous layer with reduced distortion and allowing ready cleavage in the porous layer, by decreasing a current density during anodic oxidation.
It is another object of the present invention to provide a thin-film semiconductive member having high crystallinity.
It is a further object of the present invention to provide methods for making a semiconductor substrate and a thin-film semiconductive member, capable of reducing the current density in anodic oxidation for forming a porous layer.
A first aspect of the present invention is a semiconductor substrate comprising a porous semiconductor having a porous layer with an impurity concentration distribution varying in the depth direction.
A second aspect of the present invention is a semiconductor substrate comprising a porous layer comprising a porous semiconductor containing an impurity with a content of 1xc3x971018 cmxe2x88x923or more.
A third aspect of the present invention is a semiconductor substrate comprising a porous layer provided by pore formation in an epitaxial growth layer.
A fourth aspect of the present invention is a thin-film semiconductive member formed on one surface of a supporting substrate with a porous layer provided therebetween, and separated from the supporting substrate by cleavage in the porous layer, the porous layer comprising a porous semiconductor having an impurity concentration varying in the depth direction.
A fifth aspect of the present invention is a thin-film semiconductive member formed on one surface of a supporting substrate with a porous layer provided therebetween, and separated from the supporting substrate by cleavage in the porous layer, the porous layer comprising a porous semiconductor having an impurity concentration of 1xc3x971018 cmxe2x88x923 or more.
A sixth aspect of the present invention is a thin-film semiconductive member formed on one surface of a supporting substrate with a porous layer provided therebetween, and separated from the supporting substrate by cleavage in said porous layer, the porous layer being provided by pore formation in an epitaxial growth layer.
A seventh aspect of the present invention is a method for making a semiconductor substrate comprising a variant layer forming step for forming a variant impurity layer with an impurity concentration varying in the depth direction on one surface of a supporting substrate, and a porous layer forming step for forming a porous layer by providing pores in the variant impurity layer by anodic oxidation so that the porosity in the porous layer varies in the depth direction.
An eighth aspect of the present invention is a method for making a semiconductor substrate comprising a high-impurity layer forming step for forming a high-impurity layer comprising a semiconductor having an impurity concentration of 1xc3x971018 cmxe2x88x923 or more on one surface of a supporting substrate, and a porous layer forming step for forming pores in the high-impurity layer by anodic oxidation to form a porous layer having different porosities in the depth direction.
A ninth aspect of the present invention is a method for making a thin-film semiconductive member comprising a variant layer forming step for forming a variant impurity layer with an impurity concentration varying in the depth direction on one surface of a supporting substrate a porous layer forming step for forming a porous layer by providing pores in the variant impurity layer by anodic oxidation so that the porosity in the porous layer varies in the depth direction, a step for forming a semiconductive thin film on the surface, away from the supporting substrate, of the porous layer, and a separation step for separating the semiconductive thin film from the supporting substrate by cleavage in the porous phase.
A tenth aspect of the present invention is a method for making a thin-film semiconductive member comprising a high-impurity layer forming step for forming a high-impurity layer comprising a semiconductor having an impurity concentration of 1xc3x971018 cmxe2x88x923 or more on one surface of a supporting substrate, a porous layer forming step for forming pores in the high-impurity layer by anodic oxidation to form a porous layer having different porosities in the depth direction, a step for forming a semiconductive thin film on the surface, away from the supporting substrate, of the porous layer; and a separation step for separating the semiconductive thin film from the supporting substrate by cleavage in the porous phase.
In the semiconductor substrate in the first aspect of the present invention, the impurity concentration distribution varies in the depth direction in the porous layer. For example, the porous layer is formed by anodic oxidation with a small current density and has reduced distortion.
In the semiconductor substrate in the second aspect of the present invention, the impurity concentration in the porous layer is 1xc3x971018 cmxe2x88x923 or more. Thus, the porous layer is formed by anodic oxidation with a small current density and has reduced distortion.
In the semiconductor substrate in the third aspect of the present invention, the porous layer is provided by pore formation in an epitaxial growth layer. Thus, the porosity in the direction parallel to the surface becomes readily uniform, or the porous layer may be composed of a semiconductor having a high impurity concentration.
The thin-film semiconductive member in the fourth aspect of the present invention has a porous layer having a variant impurity concentration in the depth direction. The porosity in the porous layer varies with the impurity concentration. The porous layer may be formed by anodic oxidation with a reduced current density and has reduced distortion. The thin film semiconductive member has high crystallinity.
The thin-film semiconductive member in the fifth aspect of the present invention has a porous layer having an impurity concentration of 1xc3x971018 cmxe2x88x923 or more. The porous layer may be formed by anodic oxidation with a reduced current density and has reduced distortion. The thin film semiconductive member has high crystallinity.
The thin-film semiconductive member in the sixth aspect of the present invention has a porous layer provided by pore formation in an epitaxial growth layer. Thus, the porosity in the direction parallel to the surface becomes readily uniform, or the porous layer may be composed of a semiconductor having a high impurity concentration.
In the method for making a semiconductor substrate in the seventh aspect of the present invention, a variant impurity layer with an impurity concentration varying in the depth direction is first formed on one surface of a supporting substrate. Next, the variant impurity layer is converted into a porous layer having a variant porosity in the depth direction.
In the method for making a semiconductor substrate in the eighth aspect of the present invention, a high-impurity layer comprising a semiconductor having an impurity concentration of 1xc3x971018 cmxe2x88x923 or more is first formed on one surface of a supporting substrate. Next, the variant impurity layer is converted into a porous layer having a variant porosity in the depth direction.
In the method for making a thin-film semiconductive member in the ninth aspect of the present invention, a variant impurity layer with an impurity concentration varying in the depth direction is formed on one surface of a supporting substrate. Next, the variant impurity layer is converted into a porous layer having a variant porosity in the depth direction. A semiconductive thin film is formed and is separated from the supporting substrate by cleavage in the porous phase.
In the method for making a thin-film semiconductive member in the tenth aspect of the present invention, a high-impurity layer comprising a semiconductor having an impurity concentration of 1xc3x971018 cmxe2x88x923 or more is formed on one surface of a supporting substrate. Next, the variant impurity layer is converted into a porous layer having a variant porosity in the depth direction. A semiconductive thin film is formed and is separated from the supporting substrate by cleavage in the porous phase.